Electric water heaters are provided to deliver hot water for domestic or commercial use and such typically is comprised of a steel tank provided with resistive heating elements which are turned on and off as hot water is used or the water heater is in a stand-by state. Thermostats are used to sense the temperature of water in the tank and as hot water is drawn out the cold water is introduced, usually in the bottom of the tank, and this causes a temperature change in the water in the tank and which is detected by the thermostats as the water temperature drops below the set point temperature value of the thermostat sensors. This change in water temperature causes power to be connected to the resistive heating elements until the water heats to a preset temperature value which is sensed by the thermostat which then cut-off the supply to the heating elements.
Demand for hot water, and hence power from the grid supplying the power, typically occurs in predetermined cycles when there is a common need for people to use hot water and this being between 6:00 a.m. to 9:00 a.m. and 6:00 p.m. to 9:00 p.m. which periods are commonly referred to as peak demand periods. The load on the power supply grid is also maxed during these peak demand periods and consequently the cost to produce electricity during such periods is high has is the cost to the consumer. Various methods have been developed in an attempt to save energy and minimize costs to the consumer and the utility providing the power from a power grid that it controls.
One known method is to use programmable timers to control the connection of power to the resistive heating elements and this being done usually during non-peak periods where the cost of the energy is at its lowest. Three element water heaters are known wherein each of the resistive heating elements are automatically controlled during predetermined time periods of a day outside the peak electrical demand hours whereby to cut down on the power demand by the water heater during peak periods. The resistive heating elements of such water heaters usually have different power ratings. Such water heaters also have a pre-programmed control circuit which operates the resistive heating elements regardless of the condition of the electrical power grid supplying the water heaters with power. Reference is made to U.S. Pat. No. 4,948,948 which relates to a water heater with multiple heating elements having different power ratings and operated by a control/timer circuit which operates switches to switch the elements on an off during programmed times in a day. Reference is also made to U.S. Pat. No. 7,620,302 which also relates to a three element high efficiency domestic hot water heater wherein the middle element is disposed at a level which is calculated at approximately an average maximum water consumption volume drawn during a peak power demand time period and slightly above mid-level to provide a better supply volume of hot water.
Another method is to provide a high temperature water heater equipped with a mixing valve to mix the very hot water from the tank with the water from the domestic supply and thus resulting in an effective increase in the hot water capacity that can be delivered without increasing the size of the tank.
Another approach is to shut-off the entire water heater for a predetermined time during peak periods and such as not proved to be successful as the consumer may run out of hot water due to heat loss and use of hot water causing the water in the tank to cool down during the peak periods and waiting for the non-peak periods to be heated. Such an approach would require a large tank and added cost and also a need for more space for installation and further require consumer adaptation and change of the usual custom of use.
In view of the above, a need exists to provide an electric water heater and a method which is adapted for energy management and which is efficient in reducing power consumption from the power supply grid and which is also controllable by the utility to control the power demands on its power supply grid and allow it to also regulate its load frequency. There is also a need to retrofit some existing water heaters to adapt to such energy management at minimal cost.